Piezoelectric vibrator



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"ILSE um Leermcol/snc TMNS ` IFIER Mcm- AWSTAC MC WLBGND l By WI RMASNPaterited sept. 16, 1947 UNITED STATES PATENT OFFICE PIEZOELECTRICVIBRATOR Walter L. Bond, South Orange, and Warren P. Mason, West Orange,N. J., assignprs to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application August 19, 1an,serial No."407,456

4 Claims.

stantially minimized and the efficiencies of the crystal and .the systemin which it is employed yare increased.

It is known in the art to employ piezoelectric crystals to convertelectrical wave energy into compressional (or acoustic) wave energy andthe reverse.

In systems employing crystals in this manner, however, a number ofdifficulties have been encountered, among which are relatively lowefdciency, echo effects resulting from reflections from the crystalsurfaces, echo effects resulting from radiation by the crystal indirections other than a particular desired direction and unwantedresonant and transient effects in thevibratory response of the crystal.

It has been found that the above-mentioned and possibly additionalundesirable effects largely result from or are aggravated by substantialacoustic impedance or compressional wave impedance differences betweenthe crystal and the medium in which it is used.

The acoustic impedance or compressional wave impedance (Za) of amaterial is usually defined as the product of the velocity ofpropagation, V, of acoustic or compressional waves through the materialand the density of the material p, i. e.,

Za=pV (1) For convenient reference representative values of the acousticor compressional wave impedances for a number of materials which may beemployed in systems illustrative of the principles of the invention aretabulated |below:

Material (c. g. s. units) Za=pV Methyl methacrylate 2.84X105 Polystyrene2.64 X 105 Cellulose acetate 3.26X105 Moldedl phenol formaldehyde3.75X105 Urea formaldehyde 4.2 X105 India ebony 4.4 X105 X-cut quartz14.4 X105 Aluminum 13.8 X105 More par- Tungsten 83.0 X105 Water 1.44X105Steel 40.0 X105 Rochelle salt 5.76X105 yKerosene 1.04Xl05 permalloy, 15%phenol formaldehyde1 12.8 X105 85% tungsten, 15% phenol formaldehyde114.0 X105. 30% permalloy, 70% phenol formaldehyde1 4.55X105 30%tungsten, 70% phenolformaldehyde1 4.55X105 1 Percentages given are byweight.

In systems employing piezoelectric crystals to radiate or receiveacoustic or compressional waves in a liquid, steps should be taken toalleviate the effects of a mismatch of impedance between crystal andliquid.

This can best be done by inserting between the liquid and the crystal alayer of material having an acoustic or a compressional wave impedancewhich is substantially the geometric mean of the impedances of thecrystal and the liquid, the thickness of the layer of material in thedirection of propagation being substantially one-quarter wave-length ofthe frequency of the energy, or of the mean or predominant frequencywhere a band of frequencies is being propagated. An odd multiple of thequarter wave-lengths in thickness may be used provided the acoustic orcompressional waveenergy absorption (or attenuation) of the material isnot undesirably high.

Such a layer of material then acts as an impedance transformer andprovides an improvement in the impedance match between the impedance ofthe crystal and that of th'e liquid and substantially reduces reflectionlosses therebetween and therefore materially reduces echo effects andsimilar undesirable reflective phenomenawhich can otherwise arise inmany systems employing piezoelectric crystals in connection withcompressional wave transmission systems.

By way of example, for a quartz crystal, having a compressional waveimpedance of 14.4X c. g. s. units, immersed in water of 1.44X105 c. g.s. units impedance, a material having an acoustic impedance of 105 V1.44 X 14.4=4.55 X 105 c. g. s. units should be interposed between theCrystal and the liquid.

Metals on thev other hand have W absorption and high compressional waveimpedance but are highly conductive. v

It has been found possible to combine metals and plastics either inalternate layers or in the form of metallic particles suspended in aplas-` tic to obtain composite materials which will providecharacteristics intermediate those of the constituent materials. Forexample, the plastic, phenol formaldehyde, with 30 per cent by weightofV powdered Permalloy suspended therein (socalled 85 per centPermalloy, i. e. 85 per cent Ni, per cent Fe, in alloy form, being used)has a compressional wave impedance of substantially 455x105 c. g. s.units which, as mentioned above, v

is suitable for a material to act as an impedance transforming layerbetween a quartz crystal and water. The material is, furthermore,electrically non-conductive and has a lower absorption than phenolformaldehyde alone.

For Rochelle salt piezoelectric crystals employed to radiate or absorbcompressional waves in water, a material having a compressional waveimpedance of 105 \/1.44 5.76=2.88 105 c. g. s. units should lbeinterposed between the crystal and the water. From the tabulation givenabove it is seen that methyl methacrylate has substantially the desiredcompressional wave impedance. In this particular case the enclosure ofthe crystal should be moisture-proof since Rochelle salt is soluble inwater.

In general, by suspending metallic particles in a plastic, as suggestedpreviously, any compressional Wave impedance intermediate that of theplastic and that of the ,metal may be realized.v The acousticimpedance-varies directly with the proportion of metal to plastic in thecomposite material and the absorption ordissipation will vary inverselywith the proportion of metal.

For systems in which a path through a liquid is included in the energycircuit `and piezoelectric crystals are used to introduce and abstractenergy from the liquid, a liquid consisting of a mixture of water andalcohol, of water and ace-` tone or of water and ethylene glycol may beused. When the proper proportions of alcohol (12.5 volumes to 100volumes of water), acetone (10.5 volumes to 100 volumes of water), orethylene glycol (13 volumes to 100 volumes of water) are used the liquidwill have, substantially, a zero coelcient of variation of velocity withtemperature at 55 C. mean temperature as will be described in moredetail hereinafter. For such systems phenol formaldehyde is a desirableplastic to use in the manufacture of impedance matching materials of theinvention since it is not soluble in the liquids such as alcohol,acetone or ethylene glycol which it may be found desirable to use.

In many systems also it will be desirable to radiate or receive energyfrom only one side of the crystal in which case it is not only desirableto match the compressional .wave impedance on the radiating (orreceivingy side of the crystal, but it is also desirable both to matchthe impedance and to provide for absorbing energy which reaches theopposite 0r passive side, either by virtue of the electrical drive onthe crystal itself or by reception of direct or reflected energy fromthe medium in which the crystal is immersed. It should be noted thatsome of the energy received by the first-mentioned or active" side ofthe crystal will pass through the crystal to the opposite side and maycause objectionablev echoes unless the precautions, just describedabove, are taken.

A further advantage may under particular circumstances also be obtainedby deliberately damping the crystal, where it is desired to employcrystals in pulse timing circuits since an undamped crystal generatesapulse having its maximum amplitude near the center of the pulse whereas`a damped crystal will generate a pulse having maximumY amplitude nearthe. start of.y

the pulse. Reflections of the latter type of pulse are less likely tointerfere with the directly transmitted or wanted pulse signals and alsosince the .timing is best done at the start of the pulse a verysubstantial improvement in operation and yide an improved compressionalwave impedreflections of energy at the surfaces of a piezoelectricradiator or absorber surrounded by com` pressional wave conductingmedia.

Another object is to provide improved pulse timing piezoelectric crystaldevices.

Other and further objects will become apparent during the course of thefollowing description and in the appended claims.

The principles of the invention and a number of applications thereofwill be more readily understood in connection with the followingdetailed description of illustrative embodiments shown in theaccompanying drawings in which:

Fig. 1 shows a pair of piezoelectric crystals immersed in a liquidmedium and provided with compressional wave impedance corrective frontand back members;

Fig. 2 shows in detail a specific type of piezoelectric crystal mountingillustrating an application of certain principlesv of the invention;

Figs. 3A and 3B show wave trains representing compressional wave pulseswhich can be employed in piezoelectric crystal timing systems and willbe employed in explaining advantages of particular arrangements of theinvention;

Fig. 4 shows in block diagrammatic form a -simple pulse-type distancemeasuring system in which a unit of the general type illustrated by eFig. 1 may be advantageously employed to assist in timing the reflectedpulses; and

Fig. 5 illustrates the combination withl a crystal of a composite membercomprising alternate layers of plastic and metallic material thecomposite member having particular impedance and damping properties toprovide improved over-all operation of the piezoelectric crystal pulsetiming arrangements of the invention.

In more detail. Fig. 1 shows two piezoelectric crystals Il one at theleft end and the other at the right end o! a tank containing a liquid 3Etherein. The crystals can be of any of the wellknown piezoelectricmaterials such as quartz, Rochelle salt or tourmaline. Conducting leadsIll and electrodes I2 provide means `for making appropriate electricalcoupling with the crystals.

On the more central face oi each crystal a layer of material. member I4,is positioned. Member i4 can be, as was previously explained, anacoustic impedance transforming device to match the impedance of thecrystal with that of the liquid andV it can further introduce energyabsorption or damping of the crystal. if desired, as will be discussedpresently. If impedance transformation alone i-s desired, the thicknessof member I4 is preferably one-quarter wave-length (or a low odd numberof one-quarterwave-lengths) of the frequency being transmitted throughthe liquid, or of the mid-frequency (or predominant frequency) if agroup of frequencies is being transmitted. If Rochelle salt crystals areemployed,

member I4 may be a simple plastic, for example,

methyl methacrylate, since a small impedance transformation willsuffice. If the crystals employed are quartz, however, a largertransformation will be necessary and as was previously explained, memberI4 can then comprise a plastic in which metal particles are suspended,for example, phenol formaldehyde with 30 per cent of powdered Permalloy(85 per cent Ni) to match quartz and water. Alternatively, member I4 maybe a laminated member, alternate laminations being of plastic and ofmetal, respectively, as will be described in detail presently.

On the side of the crystal opposite member I4 in each case a member I6is positioned, with a second member I8 adjacent the member I8 as shownin Fig. 1. Member I6 is similar to member i4 but its function is tomatch the compressional wave impedance of the crystal to that of themember I8 rather than to that of the liquid.

One of the possible constructions suggested for member I4 can obviouslybe selected for member I6 depending upon the particular impedancematching problem presented. Member I8 is an absorber of compressionalwave energy, for example felt, the function of which is to absorb energyand thus prevent its reflection back to its associated crystal or to thecrystal at the other end of the tank. Reilections or echoes aregenerally objectionable in communication or Vmeasuring circuits as theytend to distort or obscure the desired signals or to provide misleadingsignals and the substantial elimination at the .back sides of thecrystals of reilections is highly desirable for many purposes.

In cases where it is desirable to exclude the liquid from direct contactwith the crystal and associated impedance corrective and energyabsorbing members. for example when Rochelle salt crystals soluble inwater are used, a thin membrane oi' rubber or similar material can beemployed to form a liquid tight envelope II without appreciably dampingthe action of the assembly. The effect of such a membrane from thestandpoint of impedance, if appreciable, can be taken into account inthe over-all design of the assembly.

Where the time of travel of a compressional wave, generated by onecrystal in response to electrical stimulation, through the liquid 88 tothe other crystal, is employed in timing some other phenomena. forexample, the time interval required for an energy puise to travel to adistant surface and return to its point of origin, it is convenient tobe able to adjust the distance between the crystals in tank 20. For thispurpose, one crystal can be supported through a rotatable collar 28 on arod 24 extending through a threaded bushing 28 in the side of tank 20.Rod 24 is threaded for the greater part of its length so that by turningknob 28, the longitudinal position of the crystal supported on rod .24may be adjusted over an appropriate range. Knob 28 carries a suitablescale 21 and a slidable index pointer 28 carried on a fixed rod 25 isprovided tofacilitate use of scale 21. A micrometer arrangement of anyof the types well known in the art may be employed to afford preciseadjustment, if desired.

As previously mentioned. three particularly suitable liquids for use indevices of the type illustrated by Fig. 1 are water with approximately12.5 volumes of alcohol to 100 volumes of water, 10.5 volumes of acetoneto 100 volumes of water, and 13 volumes of ethylene glycol to 100volumes l of water. These liquids all have a zero temperaturecoeillcient of velocity at 55 C. vand a slow parabolic variation ofvelocity on either side of that, For example, a rough thermostat set tokeepthe temperature to :6 C. will hold the velocity constant to one partin 3,000. Of these mixtures the one employing ethylene glycol isparticularly advantageous since it will evaporate very slowly, and ifthe mixture is taken to 40 C. it will even then freeze in only a verymushy form which will not injure the apparatus. The respectivevelocities of these Vthree mixtures are alcohol-water. 1557 meters persecond at 55 C.. acetone-water, 1565 meters per second and ethyleneglycol-water 1594 meters per second.

The velocity of a three-component mixture containing alcohol, ethyleneglycol and water can be made to vary from 1557 meters to 1594 meters bychanging the relative proportion of alcohol and ethylene glycol andstill maintain a zero coefllcient at 55 C. This is advantageous inmatching the velocity of the liquid to standard screw threads in thevariable delay circuit of Fig. l. This three-component mixture may alsobe employed to assist in the final adjustment of the impedance matchbetween the piezoelectric devices and the liquid. An impedance change asgreat as 15 per cent can thus be effected.

In Fig. 2, details of a particular form of crystal mounting, embodying anumber of principles of the invention, are shown,

Crystal 34, having electrodes 40 and conductive leads 38 to affordconvenient electrical coupling thereto, is enclosed between members 36and 32 which can be-of plastic material; for example methylmethacrylate, polystyrene, phenol formaldehyde, urea formaldehyde or thelike, which may have suspended therein metallic particles in the eventthat it is desired to impart a greater compressional wave impedance tothem. Member 32 in addition to forming part of the holder can also actas a compressione] wave impedance transformer and, if desired, -can alsocontribute compressional wave energy absorption. Members 86 and 42 canlikewise be plastics, or plastics with metal particles suspendedtherein, or alternatively they can be of laminated construction withalternate laminae of plastic and metal, and member 44 is a compressionalwave energy absorbing member, for example felt, the function of which isto absorb the energy reaching it. 'I'he assembly is held together bybolts 46 and nuts 41, which bolts may further serve to facilitatemounting the arrangement in operating position either in a tank such as20 of Fig. 1 or on a vessel, buoy, or the like for use in submarinesignaling systems. The assembly is preferably sealed to be liquid tighteither by fusing th'e edges of theplastic members together by a hot ironwhere they come together or by enclosing the assembly in a thin membrane43 of rubber or the like; If member 44 is of felt or other liquidabsorbing material it will, of course, be necessary to enclose it, atleast, in some liquid-proof enclosure, if it is to be submerged.

Where radiation or absorption from both sides of crystal 34 isdesirable, members 42 and 44 can be omitted and members 32 and 36 canfunction as impedance transformer and/cr compressional wave dampingmeans for the two sides oi.' the crystal. respectively, in substantiallyidentical manners...

The curve 48 oi Fig. 3A indicates the amplitude response with time of anundamped piezoelectric crystal of the types contemplated for use inarrangements of the invention. Obviously, reflected echoes of an energypulse resulting from such response, arriving a half pulse length or soin advance of a succeeding directly transmitted pulse can entirely maskthe initial vibrations of the latter pulse and cause a false indicationin pulse timing arrangements of the invention. However, if the crystalis damped, for example by placing in contact with it a material ofrelatively high compressional wave energy absorption (dissipation orresistance), its amplitude response can be changed to that representedby curve '50 of Fig, 3B, and combinations of echo pulses and directlytransmitted pulses will in general produce indications which aredistinguishable from single pulses of either type and (except where thetwo are in synchronism, or very nearly so) separate indications of thearrival of the two types of pulses can be obtained so that thelikelihood of erroneous and misleading indications is substantiallyreduced. It is for this reason that it is under proper circumstancesdesirable to introduce absorption or loss in the auxiliary membersassociated with the crystal to damp its action as suggested inconnection with Figs. 1 and 2 above.

In Fig. 4 a pulse-reflection type of distance measuring system isindicated in block diagram form. Oscillator 60 furnishes a sine wave topulse generators 62 and 86 which generate one pulse for each cycle ofthe sine wave. Pulse amplifier 64 amplies the'pulses furnished bygenerator 62 and actuates electroacoustic transmitter 66, to send out aseries of` acoustic pulses having the periodicity determined by theoscillator 6U. Obviously, this periodicity should be such that relectedpulses from a. surface at the greatest distance to be measured willarrive back at acoustic receiver 18 before the next succeeding pulse isemitted by transmitter 66.

Reflections 'I4 of the pulses 10 from a distant surface 'l2 are receivedin electroacoustic receiver 18, converted into electrical pulses andapplied to a vertical deflecting plate of cathode ray oscilloscope 80.The horizontal deflecting plates of the oscilloscope are connected to asweep circuit 82 which furnishes, preferably, a saw-tooth wave deectingvoltage which deects the ray of the oscilloscope across the target insynchronism with the emission of pulses by transmitter 66. Pulsegenerator 86 provides the liquid, adjustable, delay circuit 84 with aseries of pulses which are in synchronism with the pulses of generator62. The delay circuit output is supplied to the other verticaldefiecting plate of the oscilloscope and the delay circuit is adjusteduntil the pulses furnished by it are in synchronism with the pulsesfurnished by receiver 1 The adjustment'of circuit 84 required to pr ducethis synchronism is therefore a measure of the time of travel andtherefore the distance traveled by the pulses to the reflecting surface'l2l and back and the dial of the delay circuit can therefore becalibrated to read the distance directly.

Obviously, a radio transmitter and antenna and a radio receiver andantenna could be substituted for the corresponding acoustic transmitterand receiver, respectively, of Fig, 4. in which case the dial of theadjustable delay circuit should be calibrated in terms of the time oftravel of electromagnetic waves, rather than acoustic waves.

Suitable apparatus units for all component apparatus of the system ofFig. 4 are well known to the art, whether acoustic or electromagneticwaves are employed, except for the adjustable delay circuit which is, ofcourse', of the type described in detail above in connection with Fig.l.

For example, see copending application of D. Pollack, Serial No.409,600, led September 5, 1941, entitled Phase and distance measuringsystems, for a pulse reilection type of distance measuring circuitemploying electromagnetic waves.

Fig. 5 illustrates in detail the method of building up a laminatedmember for use adjacent to a piezoelectric crystal for impedancematching and damping corrective purposes,

In Fig. 5 a piezoelectric crystal 54 has a backing comprising, layers ofa plastic 52 and layers of a metal 56. The thickness of each layer is awave-length of the frequency to be used, or of the mid-frequency of theband if a band of frequencies is to be used. Assuming radiation orreception of compressional wave energy from the back of the crystal isnot desired, a layer of felt 44 or other absorbing material may be addedat the left of the laminated member as shown in Fig.v 5. Where Zoi isthe compressional wave impedance of the plastic used and Zoz is theimpedance of the metal used, the effective impedance of the laminatedstructure, ZL, is given by the following equation:

The layer of plastic next to the crystal may, of course, be part of abox-like structure in which the crystal may be assembled as for thecrystal of Fig. 2 and as suggested in connection with Fig. 2 thelaminated structure may be member 42 of Fig. 2, a member 44 of felt orother absorbing material being added if no radiation or reception fromthat side of the crystal is desired. Member 44 is preferably enclosed ina thin membrane 5| of rubber or other moisture-proof material.

Numerous other arrangements embodying the principles of the inventionand fairly within the scope thereof will occur to those skilled in theart. The scope of the invention is dened in the appended claims.

What is claimed is:

1. In an electrocompressional wave pulse transmission system, a devicefor converting energy of one type to energy of the other type comprisinga piezoelectric crystal immersed in a duid having a substantiallydifferent impedance than the crystal, a member of material adjacent anemitting or 9 receiving side of said crystal, said member having anacoustic impedance intermediate that of said crystal and the said fluid,said intermediate impedance being substantially the geometric means ofthe crystal and fluid impedances, a second member of material adjacentthe opposite side of said crystal having an impedance substantially thegeometrical mean value between that of said crystal and a third memberof energy absorbing material said third member being placed adjacent thesecond-mentioned member whereby energy is transferred between saidcrystal and said fluid with substantially no reection and energyreaching the said second member will be absorbed and troublesomereilectionsthereof will be eliminated.

2. A piezoelectric vibrator for submarine compressional wave signalingsystems comprising a quartz crystal embedded in phenol formaldehyde, thephenol formaldehyde having in suspension therein 30 per cent by weightof Permalloy powder, whereby reflection of energy between the Avibratorand the water in which it is to be used will be substantiallyeliminated.

3. In a compressional wave pulse transmission system, a. sending andreceiving device which comprises the combination of a piezoelectricvibrating member having a compressional-wave impedance exceeding that ofthe medium in which it is to be employed, a second member positionedagainst a vibrating surface of said piezoelectric member, said secondmember having a compressional-wave impedance substantially equal to thegeometric mean of the compressional-wave impedances of saidpiezoelectric member and said medium, a third member positioned againstthe surface of said piezoelectric member opposite the first-statedvibrating surface and a fourthl member positioned adjacent the saidthird member, said third member having a compressional-wave impedancewhich is substantially equal to the geometric mean of thecompressional-wave impedances of said piezoelectric member and saidfourth member, said fourth member being of a Y material which absorbssubstantially all compressional-wave energy reaching it, the energyabsorption of said second, third and fourth members collectively beingsufficient to dampen the response of said piezoelectric member to apulse of energy so that .said response will be of maximum amplitudeinitially and will rapidly decrease, whereby unwanted reflections ofenergy in said system will be substantially reduced and interferencefrom such unwanted reections as may occur will be far less troublesome.

4. In an. electrocompressional-wave delay circuit for use in pulsetiming systems, said circuit being of the'type in which an electricalenergy pulse is converted into a, compressional-wave energy pulse at afirst point in a compressonalwave transmitting medium of knownpropagation characteristics and reconverted into an electrical energypulse at a second point a. known distance from said rst point andthetime of ,travel of said pulse through said transmitting medium isemployed as a standard of comparison by which the 10 pulse travel timeof another pulse following a path of unknown length is measured, thecombination of an electrocompressional-wave vibrating member having acompressional-wave impedance substantially different from the impedanceof said medium, means adjacent one vibrating surface of said member foreffecting an impedance match between said member and said medium, saidmeans comprising a second member of a material having acompressional-wave irnpedance which is substantially the geometric meanof the impedance of said first-mentioned member and said medium, meansadjacent a second surface of said first member for effecting thetransfer of energy from said second surface without reflection therefromand for absorbing suchenergy, said means comprising a third memberplaced adjacent the second surface of said rst member and substantiallymatching the impedance of said first-mentioned member and a fourthmember of compressional-wave energy absorbing material placed adjacentsaid third member to absorb energy reaching said third member, and meansfor conditioning the response of said vibrating member to energy pulsesto prov-ide response of initial maximum amplitude and rapidly decreasingamplitude, said laststated means comprising adequate `clampingproperties4 in one or more of the said four members whereby unwantedreflections of the energy pulses in the delay circuitl are largelyeliminated and the interfering properties of such unwanted reflectionsas may still be obtained are substantially reduced.

WALTER L. BOND. WARREN P. MASON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNrrED STATES PATENTS Number Name Date 2,138,036 Kunze Nov. 29, 19382,248,870 Langevin July 8, 1941 1,619,854 Crossley Mar. 8, 19271,732,029 Round Oct. 15, 1929 1,460,032 Moore June 26, 1923 2,263,902Percival Nov. 25, 1941 2,283,285 Pohlman May 19, 1942 2,207,656Cartwright et al July 9, 1940 1,677,945 Williams July 24, 1928 2,384,465Harrison Sept. l1, 1945 FOREIGN PATENTS Number Country Date 174,355Great Britain Mar. 8, 1923 466,212 Great Britain May 21, 1937 OTHERREFERENCES Page 223 of Handbook of Plastics by Simon'ds and Ellis.Fourth printing. D. Van Nostrand Co., 250 Fourth Ave., New York, N. Y.(Copy in Div. 15 of the U. S. Patent Oilice.)

